1. Field of the Invention
The present invention relates to an adaptive filter and an echo canceller, and particularly relates to updating of tap coefficients thereof.
2. Description of the Prior Art
In recent years, acoustic echo cancellers, line echo cancellers and the like have been researched and developed, wherein adaptive filters are used to produce echo replicas so as to cancel the echoes.
Techniques of these echo cancellers are described, for example, in the following publications:
Lit. 1: Electronic Information Communication Association Technical Research Report: April, 1989, EA89-3, entitled "Improvement on Adaptation of an Echo Canceller in a Room" PA0 Lit. 2: Electronic Information Communication Association Technical Research Report: August, 1992, EA92-48, entitled "Exponentially Weighted Step-Size NLMS Adaptive Filter Based on the Statistics of a Room Impulse Response" PA0 Lit. 3: Japanese First (unexamined) Patent Publication No. 5-304444 entitled "Unknown System Identification Method and Apparatus"
The acoustic echo cancellers in this literature use, in general, the learning identification method as one of the adaptive algorithms for achieving the echo canceling.
In each of the foregoing techniques, it is assumed that a room impulse response from a loudspeaker to a microphone is determined in advance. However, in practice, the room Impulse response varies depending on the size and state of the room, materials of the room walls, and further, arrangements of the loudspeaker, the microphone and the like. The adaptive filter using the learning identification method of the conventional technique is not sufficiently adaptive to such impulse response variation, on a practical basis.
Although the conventional adaptive filter may be effective for the acoustic echo canceller to some degree, It is not effective for the line echo canceller, which is used at a place where a transmission line delay and an echo signal coexist. The line echoes differ depending on the places of observation where the echo cancellers are provided, and further, in general, the transmission line delay exists before the presence of the actual echo, and in addition, it is frequent that the transmission delay differs depending on the state of the line connection. For this reason, it has been very difficult to suppress such echos using the conventional adaptive filter.
Further, in the foregoing publication enumerated as Lit. 3, the disclosed step size controller cannot achieve the quick adaptation of tap coefficients, and thus cannot achieve a quick echo cancellation, when the echo waveform or echo time changes.